1. Field of the Invention
This invention relates to a Karman vortex flow meter for measuring the flow rate of a fluid by detecting the frequency of Karman vortex streets or trains which are regularly produced downstream of a columnar body placed in a stream of the fluid at right angles thereto.
2. Description of the Prior Art
A Karman vortex flow meter of the type which is schematically shown in FIG. 1 is known in the art and is disclosed in, for example, Japanese Laid-Open Patent Application No. 184512/1983. The flow meter comprises a pair of columnar vortex generating bodies 2 in a pipeline 1 and a vortex detecting device 4 associated therewith. The vortex detecting device 4 essentially consists of an upper housing 21, a lower housing 22 and a vibrator 24. The columnar bodies 2 consist of an upstream columnar body 5 having a cross section which is substantially equal to that of an isosceles triangle, and a downstream columnar body 6 having a cross section which is substantially equal to that of an isosceles trapezoid, as shown in FIG. 2. They are placed in a stream of fluid at right angles thereto. The pressure fluctuations of the Karman vortices 23 which are generated around both sides of the columnar bodies 5 and 6 are transmitted to the vortex detecting device 4 through open spaces 3A and 3B. The upstream columnar body 5 has a width or base length d.sub.1 which is equal to the width or base length d.sub.2 of the downstream columnar body 6. They are spaced apart from each other by a predetermined distance as shown at a, and arranged at right angles to the direction of the fluid stream in the pipeline 1. The columnar bodies 2 provide a flow meter capable of giving results of good linearity over a wide range of measurement without causing any substantial pressure loss of fluid.
The flow meter as shown in FIGS. 1 and 2 has, however, a serious problem which will now be explained with reference to FIG. 3. The problem occurs in a transitional stage during which a stream of fluid makes a sudden change. It has been found through a visual test that small vortices 23A having a higher frequency are generated from the longitudinal edges of the upstream columnar body 5 in addition to the Karman vortices 23 which are normally generated around the downstream columnar body 6. Due to their high frequency, the vortices 23A grow more rapidly than the Karman vortices 23, especially the fluid has a very widely varying flowing speed due to pulsation. There is every likelihood that the vortices 23A may disturb the Karman vortices 23 and make it difficult to measure the flow rate of the fluid.